U.S. patent application number 11/012935 was filed with the patent office on 2005-06-30 for semiconductor substrate and method for production thereof.
This patent application is currently assigned to Siltronic AG. Invention is credited to Ammon, Wilfried von, Brunner, Josef, Deai, Hiroyuki, Grassl, Martin, Ikari, Atsushi, Matsumura, Atsuki.
Application Number | 20050139961 11/012935 |
Document ID | / |
Family ID | 34594014 |
Filed Date | 2005-06-30 |
United States Patent
Application |
20050139961 |
Kind Code |
A1 |
Brunner, Josef ; et
al. |
June 30, 2005 |
Semiconductor substrate and method for production thereof
Abstract
Hetero-semiconductor structures possessing an SOI structure
containing a silicon-germanium mixed crystal are produced at a low
cost and high productivity. The semiconductor substrates comprise a
first layer formed of silicon having germanium added thereto, a
second layer formed of an oxide and adjoined to the first layer,
and a third layer derived from the same source as the first layer,
but having an enriched content of germanium as a result of thermal
oxidation and thinning of the third layer.
Inventors: |
Brunner, Josef; (Reischach,
DE) ; Deai, Hiroyuki; (Yamaguchi, JP) ; Ikari,
Atsushi; (Yamaguchi, JP) ; Grassl, Martin;
(Emmerting, DE) ; Matsumura, Atsuki; (Yamaguchi,
JP) ; Ammon, Wilfried von; (Hochburg, AT) |
Correspondence
Address: |
BROOKS KUSHMAN P.C.
1000 TOWN CENTER
TWENTY-SECOND FLOOR
SOUTHFIELD
MI
48075
US
|
Assignee: |
Siltronic AG
Munich
DE
|
Family ID: |
34594014 |
Appl. No.: |
11/012935 |
Filed: |
December 15, 2004 |
Current U.S.
Class: |
257/616 ;
257/E21.284; 257/E21.339; 257/E21.563; 257/E21.569 |
Current CPC
Class: |
H01L 21/02236 20130101;
H01L 21/02126 20130101; H01L 21/26533 20130101; H01L 21/31658
20130101; H01L 21/02238 20130101; H01L 21/02255 20130101; H01L
21/02351 20130101; H01L 21/76256 20130101; H01L 21/02323 20130101;
H01L 21/02299 20130101; H01L 21/76243 20130101 |
Class at
Publication: |
257/616 |
International
Class: |
H01L 029/06 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 25, 2003 |
JP |
2003-430323 |
Oct 28, 2004 |
JP |
2004-314701 |
Claims
What is claimed is:
1. A semiconductor substrate, comprising a germanium doped silicon
single crystal substrate having a first concentration of germanium
and having a planar surface; an insulating oxide film formed below
the planar surface and dividing said single crystal into a first,
germanium doped silicon layer, a second, insulating oxide layer,
and a third germanium doped silicon layer, said third layer having
a higher concentration of germanium than said first layer.
2. The semiconductor substrate of claim 1, wherein said germanium
doped silicon single crystal substrate comprises a wafer processed
from a germanium doped silicon single crystal.
3. The semiconductor substrate of claim 1, wherein the first
concentration of germanium is in the range of 0.05 to 5 mol percent
based on mols of silicon and germanium.
4. The semiconductor substrate of claim 1, wherein the first
concentration of germanium is in the range of 0.2 to 1 mol percent
based on mols of silicon and germanium.
5. The semiconductor substrate of claim 1, wherein said insulating
oxide is a silicon oxide.
6. The semiconductor substrate of claim 1, wherein said second
layer has a thickness of 80 nm or more.
7. The semiconductor substrate of claim 1, wherein said third layer
is a single crystal.
8. The semiconductor substrate of claim 1, wherein said third layer
has a thickness between 1 nm and 50 nm, inclusively.
9. The semiconductor substrate of claim 1, wherein the germanium
concentration of said third layer is not less than 15 mol % and up
to 100 mol %.
10. The semiconductor substrate of claim 1, wherein said first
layer contains no COP.
11. The semiconductor substrate of claim 1, wherein said third
layer contains no COP.
12. The semiconductor substrate of claim 1, wherein the dislocation
density reaching the surface of said third layer is not more than
1.times.10.sup.5 pieces/cm.sup.2.
13. The semiconductor substrate of claim 1, wherein the roughness
of said third layer is not more than 5 nm RMS over 40.times.40
.mu.m.
14. The semiconductor substrate of claim 1, wherein the fluctuation
of the thickness of said third layer is not more than 5% or not
more than 2.5 nm.
15. The semiconductor substrate of claim 1, wherein the fluctuation
of the germanium concentration in said third layer is not more than
5%.
16. The semiconductor substrate of claim 1, further comprising a
strained silicon layer formed adjacent said third layer.
17. The semiconductor substrate of claim 1, further comprising a
germanium layer formed adjacent said third layer.
18. A method for the production of a semiconductor substrate,
comprising a) providing a wafer processed from a germanium doped
silicon single crystal ingot having a first concentration of
germanium; b) implanting oxygen ions into said wafer by ion
implantation and heat treating to form a buried oxide film, said
buried oxide film separating said wafer into a first layer below
said buried oxide film, a second layer comprising said buried oxide
film, and a third layer above said buried oxide film; and c)
thinning said third layer by thermal oxidation in an oxidizing
atmosphere to enrich the concentration of germanium in said third
layer to a second concentration higher than said first
concentration.
19. The method of claim 18, wherein prior to step b), the
concentration of germanium near the surface of said wafer is
increased by oxidizing the wafer in an oxidizing atmosphere at
elevated temperature, followed by removal of an oxide film formed
thereby.
20. The process of claim 19, wherein the oxidizing atmosphere
comprises steam, said elevated temperature is from 900.degree. C.
to the melting point of the wafer, and said oxidizing is conducted
for a period of minimally 30 minutes.
21. The process of claim 19, wherein said oxide film has a
thickness of 1 .mu.m or greater.
22. The process of claim 18, wherein said first concentration of
germanium is from 0.05 mol percent to 5 mol percent.
23. The process of claim 18, wherein said first concentration of
germanium is from 0.2 mol percent to 1 mol percent.
24. The method of claim 18, further comprising depositing a
strained silicon film above said third layer by vapor phase
deposition.
25. The process of claim 18, wherein the first concentration of
germanium and the amount of thinning are sufficient to provide a
germanium concentration in said third layer of 95 mol percent or
more, further comprising depositing onto said third layer a
germanium film by vapor phase deposition.
26. A semiconductor device prepared by processing a semiconductor
substrate of claim 1.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a hetero-semiconductor structure
possessing an SOI structure containing a silicon-germanium mixed
crystal and a method for the production thereof at a low cost with
high productivity.
[0003] 2. Background Art
[0004] In recent years, a method for effecting high speed operation
of MOSFET (Metal Oxide Semiconductor Field Effect Transistor)
integrated circuits by utilizing a technique called "strained
silicon" has been attracting attention. The strained silicon
technique consists in enhancing the mobility of electrons or
positive holes as the carrier in the channel of the MOSFET by
utilizing a silicon layer deformed so as to increase the lattice
constant.
[0005] To increase the lattice constant of the silicon layer used
for the channel, numerous methods for disposing a silicon-germanium
mixed crystal adjacent to the pertinent silicon layer have been
proposed, as disclosed in JP-A 6-252046, for example. These
conventional methods are characterized by depositing a
silicon-germanium film by vapor phase deposition on a silicon wafer
and thereafter depositing a silicon layer, again by vapor phase
deposition. Since the lattice constant of germanium is 4% larger
than that of silicon, by controlling the compositional ratio of
silicon-germanium mixed crystal, it is possible to impart the
necessary strain to the channel layer. More often than not, the
proportion of germanium is selected to be in a range of 10-30 mol
%.
[0006] This strained silicon technique may be used in combination
with the so-called SOI (Silicon On Insulator) structure, this
combination then known as SGOI (Silicon-Germanium On Insulator).
The latter combination can be produced by bonding a first substrate
prepared by depositing a multilayer film containing a
silicon-germanium mixed layer by vapor phase deposition, and a
second substrate furnished with an oxide film, and then removing
the first substrate to a certain depth by polishing or etching, as
disclosed in JP-A 10-308503. This technique is thus a combination
of the SOI technique and the strained silicon technique.
Combination with a SIMOX (Separated by IMplanted OXygen) technique
(JP-A 4-264724), another typical method for the production of SOI
wafers, has also been proposed. For example, a method for forming a
buried oxide film in a silicon-germanium mixed crystal layer by
depositing a silicon-germanium mixed crystal on a silicon substrate
and thereafter implanting oxygen ions and subsequently subjecting
the resultant composite to a high temperature heat treatment is
proposed in JP-A 9-321307. JP-2001-148473 discloses a method for
producing an SGOI wafer possessing a high germanium concentration
by the so-called ITOX technique, i.e. by decreasing the thickness
of the SOI film by oxidizing the film at high temperature, thereby
increasing the germanium concentration in the SOI film.
[0007] U.S. Pat. No. 4,975,387 discloses a method for forming a
silicon-germanium surface layer by depositing an amorphous
silicon-germanium layer on a silicon substrate and oxidizing the
resultant composite in an atmosphere of steam.
[0008] Production of silicon-germanium mixed crystals by the zone
method from a silicon raw material doped with germanium has also
been proposed. JP-A 8-143389, for example, discloses a method for
forming a bulk single crystal by adjusting the germanium
concentration in a liquid phase, thereby controlling the
concentration of germanium in the solid phase. These conventional
methods of production, however, have entailed numerous
problems.
[0009] Specifically, when a technique such as that disclosed in
JP-A 6-252046 is employed, the silicon-germanium mixed crystal
layer intended to impart strain must be sufficiently relaxed until
the lattice constant assumes a magnitude conforming to the inherent
composition. The relaxation of lattice must be relied on for
generation of dislocations. When the dislocation thus generated
extends to the region used by the relevant device, it may induce
the device to malfunction. Various measures have been proposed to
safeguard against this danger. One of the methods, as disclosed in
JP-A 6-252046 and JP-A 5-129201, comprises depositing a so-called
graded buffer layer, i.e. a layer wherein the compositional ratio
of germanium gradually increases during the formation of a
silicon-germanium mixed crystal layer by vapor phase deposition,
thereby preventing the dislocation from threading to the surface
layer. Attaining the necessary compositional ratio of germanium by
this technique necessarily results in deposition of thick films,
markedly impairing productivity, and heightening the cost of
production as a result. U.S. Pat. No. 6,039,803 discloses inclining
the main orientation of the silicon substrate by 1-8 degrees from
the normally adopted direction of <100>. However, even the
use of this method cannot be expected to attain sufficient
inhibition of dislocation, since this method entails the problem of
requiring deposition of a graded buffer layer.
[0010] The combination of the SOI structure and the strained
silicon technique which is disclosed in JP-A 9-321307 and
JP-2001-148473 does not require formation of a thick
silicon-germanium mixed crystal layer as described above. However,
the process still requires deposition of a silicon-germanium mixed
crystal layer, necessitating a complicated process of production,
and heightening the cost of production.
[0011] A method for forming an epitaxial layer by depositing an
amorphous silicon-germanium layer and subsequently oxidizing the
deposited layer in an atmosphere of steam as disclosed in U.S. Pat.
No. 4,975,387 requires a separate apparatus for the growth of the
amorphous film. Most amorphous film forming devices are susceptible
to contamination with impurities. This method, therefore, is not a
satisfactory process for the production of wafers for use in
high-speed devices desired for present and future production.
[0012] Growth of silicon-germanium mixed crystals by the
Czochralski technique or by the zone melting technique disclosed in
JP-A 8-143389 necessitates a large amount of a germanium raw
material. Since the germanium raw material is expensive, the
production of a crystal having such a high germanium concentration
in the range of 10-30 mol % required for a strained silicon
substrate has only little merit commercially. Further, an attempt
to grow from a liquid phase a single crystal containing germanium
at such a high concentration is technically difficult because
growth tends to produce dislocations.
SUMMARY OF THE INVENTION
[0013] This invention proposes a wafer having a novel layer
structure and a method for the production thereof with a view
towards solving the problems mentioned above. The wafer of this
invention has a layer on which the active region of the
semiconductor device is formed and also the substrate, both made of
silicon-germanium mixed crystals, but mixed crystals which are
widely different in concentration. The wafer of this construction
permits a strained silicon wafer of high quality to be produced at
a low cost with high productivity.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0014] One embodiment of this invention is directed to a
semiconductor substrate which is characterized by providing a first
layer formed of silicon having germanium added thereto, a second
layer formed of an oxide and adjoined to the first layer, and a
third layer formed of a silicon-germanium mixed crystal and
adjoined to the second layer. By using a silicon-germanium layer
having a low germanium concentration in the place of the ordinary
silicon layer in the first layer, it is possible to increase the
quality of the third layer and a layer formed adjoining the third
layer. It is further made possible to mass produce the second and
third layers with enhanced quality at a low cost. Thus, this
process results in an optimum technique for the production of
strained silicon wafers possessing SOI and SGOI structures which
are required by the semiconductor industry.
[0015] The invention is also directed toward a semiconductor
substrate wherein the thickness of the first layer corresponds to
the thickness of a wafer. This embodiment has all the advantages of
the first embodiment of the invention and, at the same time, the
first layer possesses sufficient mechanical strength to be used for
handling the wafer.
[0016] The invention is further directed to a semiconductor
substrate wherein the germanium concentration of the first layer is
not less than 0.05 mol % and not more than 5 mol % and preferably
not less than 0.2 mol % and not more than 1 mol %. By setting the
germanium concentration during the growth of a single crystal at a
level not lower than 0.05 mol %, it is possible to attain a
germanium concentration sufficient for the production of a strained
silicon wafer possessing an SGOI structure suitable for use as a
high-speed device, while engendering excellent crystallinity as
well. Yet, more favorable properties are obtained by setting the
germanium concentration at a level of not lower than 0.2 mol %. If
the germanium concentration exceeds 5 mol %, the overage will
induce greater frequency of dislocations during crystal pulling and
will thus render production of high quality crystals difficult. By
maintaining a germanium concentration below 1 mol %, it is possible
to facilitate the growth of crystal, obviate the necessity for
using a large amount of the expensive germanium raw material, and
reduce cost yet further.
[0017] In another embodiment, the invention is directed toward a
semiconductor substrate wherein the first layer is a single
crystal. It is thus made possible to further improve the quality of
the mixed crystal of the third layer and thus more readily permit
formation of high-speed semiconductor devices on the third layer
and on a crystal layer further deposited on the third layer.
[0018] A yet further embodiment of the invention is directed to a
semiconductor substrate wherein the second layer is formed of a
silicon oxide which is an electrical insulator. Silicon oxide
exhibits good insulating properties as compared with other oxides
such as germanium oxide or silicon oxide containing germanium at
the highest possible concentration. By forming the second layer
with silicon oxide as an electrical insulator, it is possible to
produce an SGOI wafer of high quality.
[0019] Another embodiment of the invention is directed to a
semiconductor substrate wherein the thickness of the second layer
is not less than 80 nm. By setting the thickness of the second
layer at a level of not less than 80 nm, it is made possible to
attain effective insulation and separation.
[0020] A still further embodiment of the invention is directed to a
semiconductor substrate wherein the third layer is a single
crystal. The third layer and a crystal layer subsequently deposited
on the third layer facilitate the production of semiconductor
devices.
[0021] The invention is also directed to a semiconductor substrate
as previously disclosed, wherein the thickness of the third layer
is not less than 1 nm and not more than 50 nm. If the third layer
is less than 1 nm in thickness, the low thickness will render the
production of the substrate technically difficult and will disrupt
the stability with which an additional film is formed juxtaposed
onto the third layer. Further, for the third layer, a
silicon-germanium layer having a thickness exceeding 50 nm is
neither necessary nor essential. By forming the third layer in a
thickness of not less than 1 nm and not more than 50 nm, it is made
possible to satisfy all the requirements for the semiconductor
devices of the non-depletion type, partial depletion type, and full
depletion type.
[0022] The invention is further directed to a semiconductor
substrate wherein the germanium concentration of the third layer is
not less than 15 mol % and is as much as 100 mol %. If the
germanium concentration of the third layer is lower than 15 mol %,
the low concentration will prevent the deposited strained silicon
layer from acquiring sufficient strain. Due to the presence of
germanium in the third layer, it is possible to form a
semiconductor device structure either directly on the third layer
or after the strained silicon layer has been formed adjoining the
third layer. By increasing the germanium concentration of the third
layer towards 100 mol %, it is possible to obviate the necessity of
growing a germanium bulk single crystal at a very high cost, to
form a semiconductor device directly on the third layer, and to
utilize such advantages as very high carrier mobility, a narrow
band gap, and good lattice conformity with III-V family compounds.
The formation of the third layer possessing a germanium
concentration approximating 100 mol % enables a 100 mol % germanium
layer to be formed in a vapor phase thereon while the occurrence of
dislocations by misfit is repressed. The present invention,
therefore, enjoys free selection of the thickness of germanium
crystal layer and, at the same time, gives rise to a substrate for
the formation of a germanium MOS semiconductor.
[0023] The invention is also directed toward a semiconductor
substrate wherein the first layer contains no COP. The term "COP"
refers to minute hollow defects which are formed during the growth
of a silicon single crystal and which are known to have adverse
effects on a semiconductor device. To grow a crystal containing no
COP, methods which cause a defect-free region to form throughout
the entire surface of a wafer by lowering the speed of pulling a
single crystal is known. The products produced by this method are
known as so-called "perfect crystals." By using this wafer for the
first layer, it is made possible to prevent the third layer from
forming a defect therein.
[0024] In a yet further embodiment, the invention is directed to a
semiconductor substrate characterized by the fact that the third
layer contains no COP. As a result, the deposition of an additional
layer in semiconductor fabrication processes results in a high
yield of operable devices.
[0025] The invention is also directed to a semiconductor substrate
characterized by a dislocation density reaching the surface of the
third layer of not more than 1.times.10.sup.5 pieces/cm.sup.2.
[0026] Production of the most advanced SGOIs known to date relies
on relaxation of the lattice of a heteroepitaxially grown layer.
This method causes the density of dislocations reaching the device
forming layer to increase. In contrast, the inventive wafer has no
need for heteroepitaxial growth, and therefore achieves a low
dislocation density which is a considerable advantage in the
production of a semiconductor device.
[0027] The invention is also directed to a semiconductor substrate
characterized by a roughness of the second layer of not more than 5
nm RMS over 40.times.40 .mu.m. The inventive wafer has low surface
roughness and thus does not require processing for further lowering
roughness. This low roughness is an advantage in the production of
semiconductor devices.
[0028] The invention is further directed to a semiconductor
substrate characterized by a fluctuation in the thickness of the
third layer of not more than 5%, or not more than 2.5 nm. The wafer
of the invention exhibits good uniformity of layer thickness as
compared with the most advanced methods of production known to
date, which consist of forming a thin film on an oxide film. As a
consequence of the increase in the germanium concentration of the
third layer, the content of silicon is relatively decreased and the
speed of oxidation is lowered. The variation of the rate of
oxidation with time gradually reduces non-uniformity in the
in-plane thickness and eventually converges to a uniform film
thickness throughout the entire in-plane region. This invention
utilizes this phenomenon, and the highly uniform thickness attained
thereby allows the thickness of the film to be further decreased
without impairing this uniformity. These characteristic properties
benefit the production of a semiconductor device.
[0029] The invention is also directed to a semiconductor substrate
characterized by a fluctuation of the germanium concentration in
the third layer of not more than 5%. The subject invention wafers
possess excellent in-plane uniformity of germanium concentration
due to diffusion of germanium. This characteristic property, when a
strained silicon layer is deposited on wafers such as those
described in the first embodiment of the invention, enables uniform
in-plane strain to be imparted to the deposited strained silicon
layer.
[0030] The invention is further directed to a semiconductor
substrate characterized by having a strained silicon layer
adjoining the third layer. Owing to the excellent characteristics
of the third layer mentioned above, it is possible to prepare a
strained silicon wafer possessing an embedded insulating film with
high quality at a low cost. The prominent quality and economy are
indispensable to the production of various devices of the
non-depletion type, partial depletion type, and full depletion
type.
[0031] The wafer of the invention also comprises a semiconductor
substrate, characterized by a first layer formed of silicon having
germanium added thereto, a second layer formed of an oxide and
adjoining the first layer, a third layer formed of a
silicon-germanium mixed crystal having a higher concentration of
germanium than in the first layer and adjoining the second layer,
and a fourth layer formed of germanium. Such germanium substrates
can be prepared by the process of the invention without requiring
the growth of a germanium bulk crystal which entails high cost. The
germanium can be formed as a fourth layer on the semiconductor
directly, and can utilize advantages such as very high carrier
mobility, narrow band gap, and excellent lattice doping with III-V
group compounds. When a third layer having approximately 100 mol %
germanium concentration is produced, generation of misfit
dislocations is inhibited and at a 100 mol % germanium layer can be
formed by vapor deposition. As a result, the thickness of germanium
crystal can be freely selected and it may be used as a substrate
for germanium MOS semiconductors.
[0032] The invention is also directed to a method for the
production of a semiconductor substrate, comprising:
[0033] 1) doping a silicon raw material with germanium and growing
a single crystal by the Czochralski method or the zone melting
method,
[0034] 2) processing the single crystal into wafers,
[0035] 3) implanting oxygen ions into the wafers by ion
implantation,
[0036] 4) forming a buried oxide film by high temperature heat
treatment, and
[0037] 5) thinning the crystal layer on the buried oxide film by
thermal oxidation in an oxidizing atmosphere.
[0038] According to the invention, it is possible to form an SGOI
structure with very high productivity and produce the inventive
wafers easily without requiring CVD growth of a silicon-germanium
layer. Since the process for epitaxial growth of a
silicon-germanium layer can be omitted, such problems as
dislocation and surface roughness, which are inherent in epitaxial
growth, can be alleviated. Further, the lattice constant can be
continuously varied while the occurrence of dislocations by misfit
and threading dislocations which are detrimental to a semiconductor
device is repressed. Since no need is found for the epitaxial
growth of a silicon-germanium layer, the wafer excels in uniformity
of film thickness and germanium concentration. Owing to this
outstanding uniformity, a silicon-germanium layer having a
decreased thickness of even less than 10 .mu.m is able to retain
in-plane uniformity and an extremely thin film SGOI structure which
has never been technically attained by conventional methods. For
the purpose of accomplishing a necessary germanium concentration,
the concentration of germanium used for doping at step 1) may be
adjusted, and the decrease of film thickness may be achieved by an
increase in concentration of germanium during step 5).
[0039] In the inventive process, semiconductor substrates may be
obtained by thermally oxidizing the wafer in an oxidizing
atmosphere after completion of step 2), followed by etching and
cleaning the oxide film formed on the surface, and thereafter step
3) and the following steps are performed.
[0040] By thermally oxidizing a crystal doped with germanium at a
comparatively low concentration, it is possible to increase the
content of germanium to a high concentration in the surface layer
portion of the crystal. In the thermal oxidation of a
silicon-germanium mixed crystal, silicon is preferentially oxidized
and the greater part of germanium atoms are diffused as a residue
into the substrate crystal. Since the germanium in the silicon
crystal has an extremely small diffusion coefficient, the germanium
atoms are not diffused throughout the entire substrate but allowed
to form an enriched silicon-germanium layer having germanium
concentrated in the surface layer. The present invention, by
utilizing this phenomenon, is able to form a silicon-germanium
mixed crystal layer having a high germanium concentration easily
and at a low cost. By utilizing the silicon-germanium layer thus
formed and performing the so-called SIMOX process thereon, it is
possible to produce an SGOI structure possessing a
silicon-germanium layer of good quality in the surface layer, at a
low cost, and with high productivity.
[0041] The semiconductor substrate may be oxidized by an oxidizing
atmosphere which contains steam during the process of thermal
oxidation. Generally, oxidation in an atmosphere of steam proceeds
at high velocity and thus increases productivity, since it is
capable of forming an oxide film in a short period of time and thus
further prevents diffusion of the concentrated germanium on the
surface into the bulk substrate. A layer of high germanium
concentration is formed in the surface layer.
[0042] In the process of thermal oxidation using steam, the
temperature is preferably not lower than 900.degree. C. and not
higher than the melting point, and the oxidation time is not less
than 30 minutes. If the temperature during the oxidation falls much
short of 900.degree. C., the lower temperature will result in
markedly lowering productivity because a long time is required for
increasing the germanium concentration. For the purpose of
obtaining a crystal of good quality, control of the highest
temperature for the oxidizing step below the melting point is
necessary. Since the melting point decreases as the germanium
concentration increases, the upper limit of the temperature must be
commensurate with the relevant concentration. Even when the melting
point is relatively high, the oxidation temperature is preferred to
be not higher than 1300.degree. C. When the temperature exceeds
1300.degree. C., the speed of diffusion of germanium increases and
the concentrated germanium diffuses and vanishes into the
substrate. Thus, no appreciable merit is brought by the further
increase of the temperature. When the oxidizing time is less than
30 minutes, the concentration of germanium cannot be expected to be
elevated to the desired level within the range of temperature
mentioned above.
[0043] The process of concentration of germanium during the course
of the thermal oxidation is described more specifically below. It
is known that the thickness of the oxide film ("Tox") during
thermal oxidation generally has the following relationship with the
time, t.
Tox.sup.2+A.times.Tox=B.times.t,
[0044] wherein, B denotes the "parabolic velocity constant" which
is determined by the temperature, pressure, and atmosphere during
the oxidation. The distribution of germanium in the depth direction
during the course of the oxidation can be calculated by the
following formula. 1 C t = D 2 C x 2 + 0.441 T o x t C x ,
[0045] wherein, C denotes the concentration of germanium, D the
diffusion coefficient of germanium, x the depth from the interface
between the surface oxide film and the crystal, and t, the time. On
the assumption that all the germanium atoms in the
silicon-germanium layer etched by oxidation are swept into the
crystal side, the boundary condition at x=0 can be determined so as
to conserve the total amount of germanium atoms in the crystal
layer. The average germanium concentration in the surface layer
having a thickness T1 (the layer of x=0-T1) can be approximately
expressed as follows: 2 C 0 ( 1 + ( 1 - erf ( ) + 1 ( 1 - exp ( - 2
) ) ) ) ,
[0046] wherein, C.sub.o denotes the initial germanium concentration
prior to the oxidation and .alpha. and .beta. denote the following
magnitudes. 3 = 0.39 B D , = T 1 2 D t
[0047] In accordance with this formula, the optimum
time/temperature condition during the course of the oxidation can
be found. When the time of oxidation is set at 8 hours, for
example, temperatures of 1100-1150.degree. C. are capable of
increasing the germanium concentration most effectively. When the
time of oxidation is set at 2 hours, a temperature of 1200.degree.
C. increases the concentration most effectively.
[0048] It is preferably that the oxide film formed by the process
of thermal oxidation has a thickness of not less than 1 .mu.m. By
adjusting the thickness of the oxide film as mentioned above, it is
possible to form a silicon-germanium layer having high germanium
concentration sufficient for the SGOI wafer produced by the SIMOX
process.
[0049] A strained silicon film may be deposited by the technique of
vapor phase deposition onto a semiconductor substrate of the
invention. It is possible to form a strained silicon film of good
quality and increase the carrier mobility in the channel of the MOS
device thereby.
[0050] By increasing the germanium concentration in the crystal
layer above the buried oxide film to not less than 95 mol % by the
steps previously disclosed, and further depositing a germanium film
by the technique of vapor phase deposition, excellent germanium
substrates for use in producing semiconductor devices are obtained.
For example, when the third layer has a germanium concentration
approaching 100 mol %, vapor phase deposition of a 100 mol %
germanium layer thereon is possible, while the occurrence of
dislocations by misfit is repressed. The thickness of the germanium
crystal layer can be adjusted as desired, giving rise to a
substrate useful for producing germanium MOS semiconductors.
[0051] The hetero-semiconductor structure containing a
silicon-germanium mixed crystal and the method for the production
thereof which are contemplated by this invention enable an SGOI
structure to be formed with high quality at a low cost without
requiring either the vapor phase deposition of a silicon-germanium
crystal layer or the growth of an amorphous layer or a
multi-crystal layer which has been used hitherto.
EXAMPLES
[0052] The preferred mode of embodying the present invention will
be described in detail below. The germanium concentrations shown
herein below are reported in mol %. These examples are illustrative
and not limiting.
Example 1
[0053] Five single crystal bars having different compositional
ratios of germanium were grown by the Czochralski technique using
silicon and germanium as raw materials. Four of these single
crystal bars were grown without dislocation, and were sliced,
lapped, etched, polished, and cleaned in the same manner as
ordinary silicon wafers to manufacture wafers for use in the test.
One sample taken from each of these wafers was tested for germanium
concentration by means of the SIMS (secondary ion mass
spectroscopy). The results of this test and the presence or absence
of dislocations after growth are summarized in Table 1.
1TABLE 1 Crystal 1 Crystal 2 Crystal 3 Crystal 4 Crystal 5
Germanium 0.04 0.3% 0.7% 1.2% 5.2% concentration immediately after
growth Presence of No No No No Yes dislocation immediately after
growth
[0054] Subsequently, the test wafers derived from Crystals 1-4 were
individually oxidized in an atmosphere of steam at 1150.degree. C.
for 16 hours. The oxide films thus formed had a thickness of 3.1
.mu.m. These samples were processed by the so-called low dose SIMOX
process. Specifically, the samples were implanted with oxygen ions
by the use of an ion implanter at a concentration of
4.0.times.10.sup.11 atoms/cm.sup.2 and the wafers were subsequently
heat-treated in a mixed atmosphere of argon with a trace amount of
oxygen at 1350.degree. C. for 10 hours to induce formation of a
buried oxide film. These wafers were further oxidized in an
atmosphere containing oxygen so as to thin the crystal layer on the
buried oxide film to 32 nm thickness. The oxidation was performed
at a temperature not exceeding the melting point during the course
of concentration, namely 1200.degree. C. in the case of the wafers
derived from Crystals 1-3 and 1100.degree. C. in the case of the
wafer derived from Crystal 4. The surface oxide films were removed
from the wafers with dilute hydrofluoric acid. One sample each of
the wafers was tested for germanium concentration in the SGOI
layer, the surface crystal layer directly abutting the buried oxide
film, by means of SIMS. The results are shown in Table 2.
2TABLE 2 Example 1 Conditions of preoxidation Atmosphere of steam,
1150.degree. C., 16 hours Original crystal Crystal 1 Crystal 2
Crystal 3 Crystal 4 Name of Sample 1A Sample 2A Sample 3A Sample 4A
sample Germanium 1.1% 8.9% 19.5% 32.0% concentration in 32 nm SGOI
layer
[0055] Generally, the SGOI process prefers a germanium
concentration of not less than 15% from a practical point of view.
Samples 3A and 4A were found to have high germanium concentrations.
All the samples were found to have good in-plane distribution of
germanium concentration, invariably within 5%. The samples were
also tested for in-plane film thickness distribution with a
spectroscopic ellipsometer. The test yielded good results; the
fluctuation of film thickness was 2.8% and the difference between
the largest and the smallest film thickness was 0.7 nm.
[0056] Subsequently, the samples produced were subjected to the AFM
(atomic force microscopy) observation to determine the surface
roughness. The RMS surface roughness was found to be 1.9 nm over a
scanning range of 40.times.40 .mu.m.
[0057] Next, a silicon layer of a film thickness of 15 nm was
deposited on samples 3A by vapor phase deposition. The silicon
layers thus deposited were tested for Raman scattering to determine
the strains imparted therein. As a result, the silicon layer was
found to contain strains theoretically commensurate with the
pertinent germanium concentration. Thus, the strained silicon
wafers consequently obtained were found to have good quality.
[0058] Further, to determine crystallinity, samples were subjected
to cross sectioned TEM observation, which found no discernible
dislocation by misfit in the strained silicon layer forming the
uppermost surface layer and the silicon-germanium layer underlying
it. The crystal layers obtained were found to have good
quality.
[0059] The surface layer of about 0.51 .mu.m in thickness of
strained silicon was tested for threading dislocation density by
the preferential etching technique. As a result, the sample was
found to have 7.times.10.sup.4 cm.sup.-2 of threading dislocation
density.
Example 2
[0060] The wafers derived from Crystals 1-4 grown in Example 1 were
oxidized in the same atmosphere of steam as in Example 1 at
1150.degree. C. for 16 hours and subjected to oxygen ion
implantation and heat treatment for the formation of a buried oxide
film. Subsequently, the crystal layers on the buried oxide films
were thinned to 25 nm. After the formed oxide films were removed
with hydrofluoric acid, the samples were individually tested for
germanium concentration in the SGOI layer by means of SIMS. The
results are shown in Table 3.
3TABLE 3 Example 2 Conditions of preoxidation Atmosphere of steam,
1150.degree. C., 16 hours Original crystal Crystal 1 Crystal 2
Crystal 3 Crystal 4 Name of Sample 1A' Sample 2A' Sample 3A' Sample
4A' sample Germanium 1.3% 11.7% 26.2% 43.1% concentration in 25 nm
SGOI layer
[0061] As a result, it was found that not only Crystals 3 and 4 but
also Crystal 2 having a comparatively low initial germanium
concentration was capable of forming an SGOI layer of a very high
germanium concentration.
[0062] Subsequently, on the sample 3A', a silicon layer having a
film thickness of 15 nm was deposited by vapor phase deposition.
The silicon layer thus deposited was tested by Raman scattering to
investigate the strain in the silicon layer. The silicon layer was
found to contain strain in nearly theoretical amounts. Thus, the
strained silicon wafers were of good quality.
[0063] The sample was further subjected to cross sectional
observation by TEM and selective etching. The observation found no
discernible dislocation by misfit in the strained silicon layer
forming the uppermost surface layer and the silicon-germanium layer
underlying it. The sample was found to have 9.times.10.sup.4
cm.sup.-2 respectively of threading dislocation density.
Example 3
[0064] The wafers derived from Crystals 1-4 which could be grown
without dislocation in Example 1 were oxidized in an atmosphere of
steam at 1200.degree. C. for 2 hours. The formed oxide film had a
thickness of 0.9 .mu.m. These samples were subjected to oxygen ion
implantation and heat treatment for the formation of a buried oxide
film under the same conditions as mentioned above. Subsequently,
the crystal layers on the buried oxide films were thinned to 25 nm.
The temperature of oxidation during the course of thinning the film
was the same as in Example 1. After the formed oxide films were
removed with hydrofluoric acid, the samples were tested for
germanium concentration of the SGOI layer by means of SIMS. The
results are shown in Table 4.
4TABLE 4 Example 3 Conditions of preoxidation Atmosphere of steam,
1200.degree. C., 2 hours Original crystal Crystal 1 Crystal 2
Crystal 3 Crystal 4 Name of Sample 1B Sample 2B Sample 3B Sample 4B
sample Germanium 0.5% 4.0% 9.1% 15.5% concentration in 25 nm SGOI
layer
[0065] As a result, samples 1B, 2B, and 3B manufactured from
Crystals 1, 2, and 3 were found to have rather insufficient
germanium concentrations, while sample 4B manufactured from Crystal
4 was capable of acquiring a silicon-germanium layer having a
sufficiently high germanium concentration. The fluctuation of
concentration was within 5%. The fluctuation of film thickness was
2.4% and the difference between the largest and the smallest film
thickness was 0.6 n, a satisfactory magnitude. When the samples
were tested for surface roughness in the same manner as in Example
1, the RMS was found to be 1.5 nm.
[0066] Subsequently, on sample 4B, a silicon layer having a film
thickness of 15 nm was deposited by vapor phase deposition. The
silicon layer thus deposited was tested by Raman scattering to
investigating strain in the silicon layer. The silicon layer was
found to contain a nearly theoretical amount of strain. Thus, a
strained silicon wafer of good quality could be manufactured.
[0067] Further, the sample was subjected to surface TEM observation
and selective etching. The observation found no discernible
dislocation by misfit in the strained silicon layer forming the
uppermost surface layer and the silicon-germanium layer underlying
it. The threading dislocation density was found to be
3.times.10.sup.4 cm.sup.-2.
Comparative Example 1
[0068] The wafers derived from the same Crystals 1-4 of Example 1
were oxidized in an atmosphere of steam at 1350.degree. C. for 8
hours. The oxide films thus formed had a thickness of 2.4 .mu.m.
The samples were subjected to oxygen ion implantation and heat
treatment for the formation of a buried oxide film. Subsequently,
the crystal layers on the buried oxide films were thinned to 25 nm.
The temperature of this oxidation was set at 1200.degree. C. Each
of the samples was tested for germanium concentration in the SGOI
layer by means of the SIMS. The results are shown in Table 5.
5TABLE 5 Comparative Example 1 Conditions of preoxidation
Atmosphere of steam, 1350.degree. C., 8 hours Original crystal
Crystal 1 Crystal 2 Crystal 3 Crystal 4 Name of Sample 1C Sample 2C
Sample 3C Sample 4C sample Germanium 0.4% 2.7% 6.9% 11.3%
concentration in 25 nm SGOI layer
[0069] The concentration of germanium was found to be low in spite
of the use of a higher temperature and a longer time than in
Example 2. The reason for this undesirable result is that owing to
the very high temperature, the speed of diffusion of germanium was
high enough for nearly total diffusion, and thus disappearance of
germanium into the substrate.
Comparative Example 2
[0070] The wafers derived from Crystals 1-4 which could be grown
without dislocation in Example 1 were oxidized in an atmosphere of
steam at 1250.degree. C. for 20 minutes. The formed oxide films had
a thickness of 0.4 .mu.m. These samples were subjected to oxygen
ion implantation and heat treatment for the formation of a buried
oxide film under the same conditions as mentioned above.
Subsequently, the crystal layers on the buried oxide films were
thinned to 25 nm. The temperature of the oxidation was set at
1200.degree. C. After the formed oxide films were removed, each of
the samples was tested for germanium concentration in the SGOI
layer by means of the SIMS. The results are shown in Table 6.
6TABLE 6 Comparative Example 2 Conditions of preoxidation
Atmosphere of steam, 1250.degree. C., 20 minutes Original crystal
Crystal 1 Crystal 2 Crystal 3 Crystal 4 Name of Sample 1D Sample 2D
Sample 3D Sample 4D sample Germanium 0.4% 2.9% 6.9% 12.1%
concentration in 25 nm SGOI layer
[0071] All the samples acquired rather insufficient germanium
concentrations. This is because the time of oxidation was unduly
short and the concentration of germanium was low. When the
germanium concentration is low during the course of pulling a
crystal, the time of oxidation must be lengthened.
Comparative Example 3
[0072] The wafers derived from Crystals 1-4 which could be grown
without dislocation as in Example 1 were oxidized in an atmosphere
of steam at 850.degree. C. for 4 hours. The formed oxide films had
a thickness of 0.3 .mu.m. These samples were subjected to oxygen
ion implantation and heat treatment for the formation of a buried
oxide film under the same conditions as mentioned above.
Subsequently, the crystal layers on the buried oxide films were
thinned to 25 nm. The temperature of this oxidation was set at
1200.degree. C. Each of the samples was tested for germanium
conccentration in the SGOI layer by means of the SIMS. The results
are shown in Table 7.
7TABLE 7 Comparative Example 3 Conditions of preoxidation
Atmosphere of steam, 850.degree. C., 4 hours Original crystal
Crystal 1 Crystal 2 Crystal 3 Crystal 4 Name of sample Sample 1E
Sample 2E Sample 3E Sample 4E Germanium 0.4% 2.6% 6.4% 11.7%
concentration in 25 nm SGOI layer
[0073] All the samples acquired insufficient germanium
concentrations. This is because the temperature of oxidation was
unduly low and the concentration of germanium was low. When the
germanium concentration is low during the course of pulling a
crystal, the temperature of oxidation must be heightened.
Comparative Example 4
[0074] The wafers derived from Crystals 1-4 which could be grown
without dislocation as in Example 1 were oxidized in an atmosphere
of dry oxygen at 1200.degree. C. for 1 hour. The formed oxide films
had a thickness of 0.2 .mu.m. These samples were subjected to
oxygen ion implantation and heat treatment for the formation of a
buried oxide film under the same conditions as mentioned above.
Subsequently, the crystal layers on the buried oxide films were
thinned to 25 nm. The temperature of the oxidation was set at
1200.degree. C. Each of the samples was tested for germanium
concentration in the SGOI layer by means of SIMS. The results are
shown in Table 8.
8TABLE 8 Comparative Example 4 Conditions of preoxidation
Atmosphere of steam, 1200.degree. C., 1 hour Original crystal
Crystal 1 Crystal 2 Crystal 3 Crystal 4 Name of sample Sample 1F
Sample 2F Sample 3F Sample 4F Germanium 0.3% 2.3% 4.3% 5.3%
concentration in 25 nm SGOI layer
[0075] All the samples acquired insufficient germanium
concentrations. This is because the speed of oxidation was markedly
low in the atmosphere of dry oxygen as compared with the speed of
diffusion of germanium.
Comparative Example 5
[0076] The wafers derived from Crystals 1-3 which could be grown
without dislocation as in Examples 1-5 were oxidized in an
atmosphere of steam at 1250.degree. C. for 2 hours. The formed
oxide films had a thickness of 1.0 .mu.m. These samples were
subjected to oxygen ion implantation and heat treatment for the
formation of a buried oxide film under the same conditions as
mentioned above. Subsequently, the crystal layers on the buried
oxide films were thinned to 70 nm. The temperature of oxidation was
1200.degree. C. in the case of the wafer derived from Crystal 5 and
1100.degree. C. in the case of the wafers derived from Crystals 6
and 7. Each of the samples was tested for germanium concentration
in the SGOI layer by means of SIMS. The results are shown in Table
9.
9TABLE 9 Comparative Example 5 Conditions of preoxidation
Atmosphere of steam, 1250.degree. C., 2 hours Original crystal
Crystal 1 Crystal 2 Crystal 3 Crystal 4 Name of Sample 1G Sample 2G
Sample 3G Sample 4G sample Germanium 0.2% 1.3% 2.9% 5.5%
concentration in 70 nm SGOI layer
[0077] All the samples evidenced insufficient concentration of
germanium. This is because the finally completed crystal layers had
unduly large thicknesses and concentration was consequently
insufficient.
Example 4
[0078] The wafers derived from Crystals 3 and 4 which were grown in
Example 1 were subjected to oxygen ion implantation and heat
treatment for the formation of an buried oxide film. The oxidation
prior to oxygen ion implantation and removal of the oxide film were
not carried out. Subsequently, the crystal layers on the buried
oxide films were thinned by oxidation to 10 nm. The temperature of
this oxidation was set at 1100.degree. C. After the formed oxide
films were removed with hydrofluoric acid, each of the samples was
tested for germanium concentration in the SGOI film by means of
SIMS. The results are shown in Table 10.
10TABLE 10 Example 4 Original crystal Crystal 3 Crystal 4 Name of
sample Sample 3H Sample 4H Germanium 19% 32% concentration in 10 nm
SGOI layer
[0079] When the germanium concentration was high during the course
of pulling a crystal, an SGOI layer having a sufficiently high
germanium concentration could be manufactured without performing
any oxidation prior to the oxygen ion implantation.
Example 5
[0080] The wafer derived from Crystal 5 which was grown in Example
1 was oxidized in an atmosphere of steam at 1150.degree. C. for 16
hours. The formed oxide film had a thickness of 3.1 .mu.m as in
Example 1. Subsequently, the sample was subjected to the low dose
SIMOX process and the crystal layer on the buried oxide film was
thinned by the ITOX process to 5 nm. When this sample was tested
for germanium concentration in the thinned crystal layer, the
concentration was found to be 99%. After the surface oxide film was
etched, a germanium film was deposited in a thickness of 0.2 .mu.m
thereon by vapor phase deposition. When the sample was subsequently
subjected to the cross section observation by TEM to determine
crystallinity, it was found that the single crystal layer could be
grown without dislocation.
[0081] While embodiments of the invention have been illustrated and
described, it is not intended that these embodiments illustrate and
describe all possible forms of the invention. Rather, the words
used in the specification are words of description rather than
limitation, and it is understood that various changes may be made
without departing from the spirit and scope of the invention.
* * * * *